820-00165 no backlight.

[gcain]

Liquid damaged.
bklt bins burnt off on the LVDS connector, replaced it, twice.

LED Driver had a burnt out pin 8, replaced it too.

Still no backlight. I'm getting ~8.5v on 3 & 4 of J8300.

 

[dukefawks]

Feedback trace to ball A5 checked? BKL_EN and PWM present?

 

[gcain]

Yes, I checked the trace when I replaced the LED Driver.

BKL_EN and PWM are missing, one both boards.


[SUB]I have three of them that have come back now with all similar problems.
Could it be a bad batch of LED Drivers?[/SUB]

I'm guessing pin 1 of R7704 shouldn't measure 0.000 in diode mode?

 

[dukefawks]

Is there even an image?

 

[gcain]

Definitely an image.

 

[dukefawks]

Did you check the fuse?

 

[gcain]

Yeh, I did.

Check it every now and then in case I blow it while working as I have done once before.

 

[dukefawks]

If there really is 8.5V on the backlight rail the BKL_EN must be present as it is a simple resistor devider off that rail. Double check and don't assume anything.

 

[gcain]

I just noticed that I am getting an unstable reading on D7701

It goes from 8.59 down to about 6v then back up again. Ever now and then it seems to drop to 0v.

Any idea what that means?

 

[dukefawks]

Pin 3 of Q7706? You are measuring with an LCD connected I hope.

 

[gcain]

Pin 3 of Q7706? You are measuring with an LCD connected I hope.

Q7706::3 = 2.843v

D7701 is stable now. I measured it several times before to make sure it was fluctuating....

These three backlight boards are sending me loopy.

 

[dukefawks]

Then we are back to the simple resistor divider on BKL_EN so that must be present. BKL_PWM present too?

 

[gcain]

BKL_PWM with a DMM is 1.46v

 

[dukefawks]

Come on, I also need to know if BKL_EN is there of course.....

 

[gcain]

It is, at 2.84v @ R7715

 

[dukefawks]

PWM is too low. Have you done a PRAM reset? Is SMC_LID high?

 

[gcain]

SMC_LID is 3.3

I have performed an PRAM reset.

I tried putting 3v on BKL_PWM to get the screen at full brightness, but nothing changed.

 

[dukefawks]

I have no other ideas. It is either a bad LP8550, badly soldered, corroded feedback trace to ball A5. There is nothing more to these.

 

[gcain]

Are bad LP8550s a thing?

The last batch (10, used 5) I bought, each one has come back with some sort of problem that looks like a backlight issue.

Another just came back with a flickering backlight....

 

[dukefawks]

Nope, never had an issue with them. Check your work and always inspect the feedback trace.